1. Field of the Invention
The present invention relates to an electrode assembly and a secondary battery having the same, and more particularly, to a secondary battery that inhibits corrosion and has sufficient lifespan and overcharge characteristics.
2. Description of the Related Art
In recent years, the rapid development of small and lightweight portable electronic devices has generated an increasing need for high-capacity, small-sized batteries. In particular, lithium ion secondary batteries can provide an operating voltage of at least about 3.6 V, which is about 3 times higher than nickel-cadmium batteries or nickel-hydrogen batteries widely used in portable electronic devices, and they have a higher energy density per unit weight than nickel-cadmium batteries or nickel-hydrogen batteries. For these reasons, research into lithium ion secondary batteries has rapidly progressed.
In a lithium ion secondary battery, electrical energy is generated due to oxidation and reduction reactions, which occur when lithium ions are intercalated/deintercalated at positive and negative electrodes. Fabrication of the lithium ion secondary battery involves forming positive and negative electrodes out of materials capable of reversibly intercalating/deintercalating lithium ions and filling an organic electrolyte or polymer electrolyte between the positive and negative electrodes.
The lithium ion secondary battery includes an electrode assembly in which a negative electrode plate and a positive electrode plate with a separator interposed therebetween are wound in the form of a jelly-roll, a can for containing the electrode assembly and an electrolyte, and a cap assembly mounted on the can.
Conventionally, a single or multiple polyolefin micro-porous polymer layer formed of at least one of polypropylene (PP) and polyethylene (PE) was used as the separator. However, since the polyolefin micro-porous polymer layer serving as the separator has a sheet or film shape, when heat is generated due to internal shorting or overcharge, pores may be clogged and the film-type separator may shrink.
Accordingly, when the film-type separator shrinks due to heat generated in the lithium ion secondary battery, portions of the positive and negative electrodes, which are not separated by the shrunk separator, are brought into contact with each other, thereby causing ignition, bursting, or explosion.
In order to make up for these weak points in the film-type separator, a considerable amount of research has focused on forming a ceramic separator using a porous layer formed of a combination of a binder and a ceramic material, such as silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2), or titanium oxide (TiO2).
In this case, the ceramic separator may make up for the fusion and shrinkage of a film-type polyolefin separator at a high temperature of about 120° C. or higher. As a result, there is a growing tendency to use both a conventional film-type separator and a ceramic separator.
However, when both the film-type separator and the ceramic separator are used, an additional material is required, thereby increasing fabrication costs. In addition, it is necessary to consider compatibility between the film-type separator and the ceramic separator as an additional quality control item in the fabrication process. Furthermore, a lithium ion secondary battery must be designed in consideration of the designs of the respective separators and a correlation between the film-type separator and the ceramic separator. As a result, material, design, and fabrication costs are increased.
Therefore, various methods for replacing the film-type separator by only a ceramic separator have been developed and proposed in order to overcome the thermal vulnerability of a conventional film-type separator. In this case, however, a ceramic layer needs to be made denser so that internal short-circuiting may be prevented using only a ceramic separator. Accordingly, the ceramic layer must be formed of ceramic powder with a small particle size.
However, when ceramic power with a small particle size is used, the ceramic layer becomes apt to absorb moisture from the atmosphere.
Also, when the ceramic powder is too dense, it hinders smooth movement of lithium ions, which may shorten the lifespan of the lithium ion secondary battery. As a result, the pursuit of a high degree of safety has required the sacrifice of reliability.
Therefore, in order to replace a conventional film-type separator by a ceramic separator, it is necessary to optimally design a lithium ion secondary battery so as to ensure its safety without diminishing its reliability.